1. Field of the Invention
The present invention relates to a seat back structure of vehicle seat provided with a movable headrest. In particular, the invention is directed to a seat back structure of vehicle seat of the type comprising a headrest movably provided on the seat back and a pressure receiving element provided in the seat back and operatively connected with the headrest, with the arrangement thereof being such that when a rear-end collision occurs and a backward load is applied from a seat occupant to the pressure receiving element under a backward inertia of that seat occupant, the headrest is forcibly displaced forwardly and upwardly toward a point adjacent to a head of the seat occupant.
2. Description of Prior Art
Disclosed and known for example from the Japanese Laid-Open Patent Publication No. 11-34708 is a seat back structure of the foregoing kind which is designed for protecting a seat occupant's head against a rear-end collision. In brief, in the case of rear-end collision, the body of seat occupant is naturally displaced backwardly of the seat under a backward initial of the seat occupant, which in turn applies a corresponding load (or impact load) to a given pressure receiving element provided in the seat back. At this moment, with that pressure receiving element being moved backwardly, a headrest is forcibly at once displaced forwardly and upwardly to a point adjacent to a head of the seat occupant.
According to this kind of emergency active headrest, a biasing means, or a tension spring for example, is provided between the pressure receiving element and the headrest. Normally, the headrest is biasingly retained by such biasing means toward a lower home position where it receives a head of seat occupant in an ordinary manner. The biasing means in this headrest system, or the tension spring for example has a predetermined biasing force sufficient to withstand a weight of the seat occupant's upper body portion as well as a normal load applied from that upper body portion, unless an impact load is applied thereto in a rear-end collision case by a greater force than the seat occupant's upper body portion weight and the normal load. Hence, only when such great impact load is applied to the seat back, the pressure receiving element is moved backwardly against the biasing force of the tension spring, so that the headrest is displaced from the lower headrest home position in forward and upward directions towards a predetermined upwardly displaced point of headrest.
With such arrangement, when the rear-end collision occurs, the headrest is quickly displaced toward a head of seat occupant being concurrently displaced forwardly, thereby insuring to keep a smallest possible space between the seat occupants head and headrest, and therefore it is possible to effectively reduce a degree of subsequent backward inclination of the seat occupant's head, wherein such subsequent backward inclination of seat occupant's head is quickly followed by forward displacement of that particular head under a backward inertia of the seat occupant, as is known. Accordingly, in this prior art, an abrupt backward inclination of the seat occupant's head can be prevented assuredly, and it is possible to minimize injury of his or her neck portion or whiplash injury.
In the above-described known headrest system, the pressure receiving element, adapted for receiving an impact load caused by backward inertia of the seat occupant as stated above, is disposed at a point corresponding to a dorsal part of the seat occupant.
In general, when a seat occupant assumes normal seating posture on a seat, his or her lumbar part is in a substantially full contact with the surface of seat back, while by contrast, his or her dorsal part is in most cases positioned away from the seat back a certain distance. Thus, under a backward inertia of the seat occupant in the case of actual rear-end collision, the dorsal part of the seat occupant has to move backwards a distance corresponding to the afore-said certain distance between the seat back and that particular seat occupant's dorsal part, and then reach the pressure receiving element, at which moment, the pressure receiving element is pressed by that backwardly moved seat occupant's dorsal part and forcibly moved backwardly to cause forward and upward displacement of the headrest. This means that, upon occurrence of the rear-end collision, the seat occupant's dorsal part is moved backwardly the aforementioned distance and struck against the pressure receiving element. Consequently, it is more likely than not that the seat occupant's dorsal part might be suffered from or damaged by an excessive impact, due to its being strongly struck against the pressure receiving portion, which leaves a problem in this kind of active headrest system.
An example of solution to this problem is found in the Japanese Laid-Open Patent Publication No, 2000-325179 which teaches such head protection arrangement that, in a rear-end collision case, upon a seat occupant's lumbar part being pressed against a pressure receiving element, a headrest is immediately displaced upwardly to receive a head of the occupant.
This prior-art arrangement is indeed effective in view of the foregoing statement that the seat occupant's lumbar part is in a substantially full contact with the surface of seat back at the time when the seat occupant assumes normal seating posture on a seat. In other words, in the case of rear-end collision, such arrangement does not require undesired long moving distance of the lumbar part towards the pressure receiving element in the seat back, but permits that lumbar part to quickly reach and push the pressure receiving element. Hence, this lumbar active type of head projection system insures to lessen an impact applied from the pressure receiving element to the seat occupant's lumbar part.
In this sort of head protection system, a biasing means is provided to give a predetermined biasing force to both of the headrest and pressure receiving element, and generally stated, the biasing force is always imposed and active on all operative elements which are to be moved with every movement of the headrest, including that particular pressure receiving element and headrest. Consequently, in the rear-end collision case, it is necessary that a pressure overcoming such biasing force be continuously applied from the seat occupant's lumbar part to the pressure receiving element in order to cause upward displacement of the headrest to a given point for receiving seat occupant's head and also stably retain the headrest at that given point. However, the problem is that, in most of seat back used in this kind of seat, the upper region thereof for supporting the dorsal part of seat occupant is more elastic and cushiony than the lower region thereof for supporting the lumbar part of seat occupant, and therefore, the dorsal part of seat occupant will be more deeply sunk backwards into the upper region of seat back than his or her lumbar part. In that instance, the seat occupant's seating posture is naturally changed to shift his or her weight in a direction from the lumbar part to the dorsal part, which will possibly reduce the degree of load applied to the pressure receiving element from the seat occupant's lumber part. Hence, as far as the present prior art relies only on a pressure from the seat occupant's lumbar part to the pressure receiving element, it is likely that, if that pressure is lower than a predetermine degree, the headrest will not reach the foregoing given point for supporting seat occupant's head and will not be retained at that point for safety purpose, neither.
Consequently, for the foregoing conventional seat back structure, it is possible that an excessive impact will be caused in the dorsal part of seat occupant at the time of commencement of the pressure receiving element to receive that dorsal part, and further, the seat occupant's lumar part will not sufficiently be pressed against the pressure receiving element, with the result that the pressure receiving element will not be moved enough to cause the headrest to displace to the point for supporting the seat occupant's head.